High-Dimensional Bell Test without Detection Loophole

Violation of Bell???s inequalities shows strong conflict between quantum mechanics and local realism. Loophole-free Bell tests not only deepen understanding of quantum mechanics, but are also important foundations for device-independent (DI) tasks in quantum information. High-dimensional quantum systems offer a significant advantage over qubits for closing the detection loophole. In the symmetric scenario, a detection efficiency as low as 61.8% can be tolerated using four-dimensional states and a four -setting Bell inequality [Phys. Rev. Lett. 104, 060401 (2010)]. For the first time, we show that four-dimensional entangled photons violate a Bell inequality while closing the detection loophole in experiment. The detection efficiency of the four-dimensional entangled source is about 71.7%, and the fidelity of the state is 0.995 ?? 0.001. Combining the technique of multicore fibers, the realization of loophole-free high -dimensional Bell tests and high-dimensional quantum DI technologies are promising.